The invention relates generally to cathodes for vacuum tubes and more particularly to field emitter array (FEA) cathodes for use with traveling wave tube (TWT) amplifiers or other electron devices.
An FEA generally comprises two closely spaced surfaces. The first, an emitter surface, has a large number of pyramid like shapes formed thereon. The second, a grid surface, is generally a metal sheet disposed above the emitter surface and electrically insulated therefrom. The grid generally has apertures disposed above the tips of the pyramids so that electrons emitted from the pyramid tips pass through the apertures when the grid is biased in a positive sense relative to the emitter pyramids.
The separation between the emitting surface and the grid is generally on the order of microns so that low grid voltages induce large emission currents. The emitted electrons may be accelerated and formed into a beam by standard techniques.
The FEA is now being utilized in many electron devices due to its inherent advantages over thermionic cathodes. Among these advantages are: (a) higher emission currents; (b) lower power requirements (c) less expensive fabrication and (e) easier interfacing with integrated circuits. However, despite the existence of the above-described advantages the utility of the FEA in microwave and millimeter amplifiers has been limited by two factors. First, the strong dependence of the emitted current on the emitter tip shape coupled with the difficulty of controlling tip shape results in poor point-to-point emission uniformity over the surface of the FEA. Second, residual gas absorbtion/desorption by the tips results in an emission current that is unstable and non-reproducible at a fixed grid voltage.